I'm having this same issue - I requested a copy of my registration key (using the online form), and the email I was sent contained 3 (two for v4 and one for v5, from the looks of it). I tried all to register v5.3 with all three, though, and each reports that it's a key for a previous version.
The v5 key should work. Make sure that the registered user name and key appear exactly as they do in the email. If you continue to have trouble, go to ClearContext > About > Email Support and send us the data in that email along with a screen print of the registration screen with your registration details entered into it.
On a related note, I just downloaded and installed the software from the website and it shows in Outlook as "Version 5.3.0 Beta Personal", which seems odd.
The current Personal release is still in beta as we finalize it. Thanks for the feedback.